This invention relates to solutions of polyimidesiloxanes in butyrolactone. In particular, it relates to polyimidesiloxanes that are soluble in butyrolactone, but are not soluble in diglyme.
Because of their high-temperature stability, low dielectric constant, and excellent physical properties, polyimidesiloxanes are desirable polymers for use in making circuit boards and for coating wire and cable, as well as many other applications that require those properties. For many of these applications, the polyimidesiloxane must be applied from a solution. Diglyme (diethylene glycol dimethyl ether) and N-methyl pyrrolidinone (NMP) are often used to form solutions of polyimidesiloxanes. However, for many applications diglyme cannot be used because it is flammable and it will not dissolve a polyimidesiloxane having a low siloxane content, which is required to give a high T.sub.g. NMP is a good solvent for polyimidesiloxanes, but because it has a high boiling point and is polar it is difficult to remove, and high temperatures and long periods of time are required to evaporate it completely.
Butyrolactone would be a desirable solvent for polyimidesiloxanes because it is less flammable than diglyme and can dissolve polyimidesiloxanes having a lower siloxane content than diglyme. Compared to NMP, butyrolactone is less polar, and therefore can be removed by evaporation more easily. However, until now, particular polyimidesiloxane polymers that are soluble in butyrolactone have not been specifically identified. In addition, in some applications, diglyme is used as a cleaning solvent, or as a solvent to apply coatings of other materials over a coating of butyrolactone. For those applications it would be necessary to have polyimidesiloxanes that are soluble in butyrolactone, but are not soluble in diglyme. However, the solubility of a particular polyimidesiloxane in a particular solvent is not a well-understood phenomena. At the present time chemistry has not advanced sufficiently to enable one skilled in the art to predict which polyimidesiloxanes will be soluble in butyrolactone, but will not be soluble in diglyme.